1. Field of the Invention
The present invention generally relates to drill bits useful for drilling oil, gas and water wells and methods for manufacturing such bits. More specifically, the present invention relates to a stabilized bi-center bit incorporating shaped polycrystalline diamond compacts which are selectively positioned about the cutting surface of either or both of the pilot and the reamer, and/or a redesign of the pilot vis-a-vis the reamer to optimize force balancing.
2. Description of the Prior Art
A significant source of many drilling problems relates to drill bit and string instability, of which there are many types. Bit and/or string instability probably occurs much more often than is readily apparent by reference to immediately noticeable problems. However, when such instability is severe, it places high stress on drilling equipment that includes not only drill bits but also downhole tools and the drill string in general. Common problems caused by such instability may include, but are not limited to, excessive torque, directional drilling control problems, and coring problems.
One typical approach to solving these problems is to over-design the drilling product to thereby resist the stress. However, this solution is usually expensive and can actually limit performance in some ways. For instance, one presently commercially available drill bit includes reinforced polycrystalline diamond compact ("PDC") members that are strengthened by use of a fairly large taper, or frustoconical contour on the PDC member. The taper angle is smaller than the backrake angle of the cutter to allow the cutter to cut into the formation at a desired angle. While this design makes the PDC cutters stronger so as to reduce cutter damage, it does not solve the primary problem of bit instability. Thus, drill string problems, directional drilling control problems, and excessive torque problems remain. Also, because the PDC diamond table must be ground on all of the PDC cutters, the drill bits made in this manner are more expensive and less resistant to abrasive wear as compared to the same drill bit made with standard cutters.
Another prior art solution to bit instability problems is directed toward a specific type of bit instability that is generally referred to as bit whirl. Bit whirl is a very complicated process that includes many types of bit movement patterns or modes of motion wherein the bit typically does not remain centered within the borehole. The solution is based on the premise that it is impossible to design and build a perfectly balanced bit. Therefore, an intentionally imbalanced bit is provided in a manner that improves bit stability. One drawback to this method is that for it to work, the bit forces must be the dominant force acting on the bit. The bits are generally designed to provide for a cutting force imbalance that may range about 500 to 2000 pounds depending on bit size and type. Unfortunately, there are many cases where gravity or string movements create forces larger than the designed cutting force imbalance and therefore become the dominant bit forces. In such cases, the intentionally designed imbalance is ineffective to prevent the bit from becoming unstable and whirling.
Yet another attempt to reduce bit instability requires devices that are generally referred to as penetration limiters. Penetration limiters work to prevent excessive cutter penetration into the formation that can lead to bit whirl or cutter damage. These devices may act to prevent not only bit whirl but also prevent radial bit movement or tilting problems that occur when drilling forces are not balanced.
As discussed in more depth hereinafter, penetration limiters should preferably satisfy two conditions. Conventional wisdom dictates that when the bit is drilling smoothly (i.e., no excessive forces on the cutters), the penetration limiters must not be in contact with the formation. Second, if excessive loads do occur either on the entire bit or to a specific area of the bit, the penetration limiters must contact the formation and prevent the surrounding cutters from penetrating too deeply into the formation.
Prior art penetration limiters are positioned behind the bit to perform this function. The prior art penetration limiters fail to function efficiently, either partially or completely, in at least some circumstances. Once the bit becomes worn such that the PDC cutters develop a wear flat, the prior art penetration limiters become inefficient because they begin to continuously contact the formation even when the bit is drilling smoothly. In fact, a bit with worn cutters does not actually need a penetration limiter because the wear flats act to maintain stability. An ideal penetration limiter would work properly when the cutters are sharp but then disappear once the cutters are worn.
Another shortfall of prior art penetration limiters is that they cannot function of the bit is rocked forward, as may occur in some types of bit whirling or tilting. The rear positioning of prior art penetration limiters results in their being lifted so far from the formation during bit tilting that they become ineffective. Thus, to be most effective, the ideal penetration limiter would be in line with the cutters rather than behind or in front. However, this positioning takes space that is used for the cutters.
While the above background has been directed to drill bits in general, more specific problems of bit instability are created in the instance of the bi-center bit. Bi-center bits have been used sporadically for over two decades as an alternative to undereaming. A desirable aspect to the bi-center bit is its ability to pass through a small hole and then drill a hole of a greater diameter. Problems associated with the bi-center bit, however, include those of a short life due to irregular wear patterns and excessive wear, the creation of a smaller than expected hole size and overall poor directional characteristics.
As in the instance of conventional drill bits, many solutions have been proposed to overcome the above disadvantages associated with instability and wear. For example, the use of penetration limiters has also been employed to enhance the stability of the bi-center bit. However, the prior art has not addressed the difficulties associated with the placement of such penetration limiters to properly stabilize the bi-center bit, which by its design, is inherently unstable. Penetration limiters in more traditional applications have been simply placed behind multiple cutters on each blade and only the exposure of the cutters above the height of the penetration limiter was felt critical to producing proper penetration limiter qualities. Additional considerations, however, are involved with the placement of shaped cutters on a bi-center bit which must contemplate the cutting force of both the reamer and the pilot bit.
As a result of these and other proposed problems, the bi-center bit has yet to realize its potential as a reliable alternative to undereaming.